1. Field of the Invention
The present invention relates to improvements in the design of screws employed in the plastics processing industry to melt and convey plastic for extrusion processes.
2. Summary of Prior Art
Plastic or polymer materials are prepared for injection or extrusion by melting and mixing of plastic or polymer material (usually initially in pellet or powder form) and then conveying the melted and mixed material at pressure for either injection into a mold or for extrusion processes. Typically, this is accomplished by a “flighted” screw rotating in a heated barrel, through which the plastic or resin material passes en route to the mold or extrusion apparatus. The rotation of the screw and action of the flights upon the plastic pellets, combined with heat from the barrel, melt, mix and convey the plastic for injection or extrusion. Accordingly, screws are generally divided into “feed” sections (where the unmelted plastic or resin pellets enter the barrel), “melt” sections (where melting of the pellets takes place), and “metering” sections (where the quantity of melted plastic is delivered to the mold or extrusion apparatus.
Generally speaking, extrusion is the more critical process because uniformity of the melted plastic material and consistent delivery rates to the extrusion apparatus are essential to quality control in the extrusion process. In the plastics industry, extruding plastic resins efficiently is critical to the product quality, productivity, and energy efficiency. Achieving efficiencies that are closer to enthalpy or greater pounds (of plastic or resin) per hour per horsepower (power used in the extrusion process) improves all aspects of extruding resins. Accordingly, much design effort has been expended to develop optimum screw designs to accomplish these functions in an efficient and effective manner.
In the prior art, starting with Maillefer (U.S. Pat. No. 3,358,327), screw designs had an auxiliary or “barrier” flight that emulates the progression of melting in a conventional “square pitch” screw design (in which the pitch between successive turns of the flight is the same as the diameter of the shaft). In a conventional design, the solids bed (unmelted pellets or powder) decreases in width and the melt pool (molten plastic) increases in width along the length of the screw. In Maillefer, a second or barrier flight that starts on the leading side and (due to an elongated lead) catches up to the trailing side of the main flight in a predetermined distance. This creates primary and auxiliary “channels” between the flights that separates the solids from the melt and simulates the progression of melting in a conventional screw design. This narrowing of the primary channel decreases the solids bed width and therefore reduces the effective melting area.
Wheeler (U.S. Pat. No. 4,341,474) mistakenly decreases the primary channel width and increases the auxiliary channel width, thereby sacrificing melting efficiency for melt-conveying efficiency. The premise was to decrease the potential for material hang-up. Because the pressure established for melting is more than sufficient to deliver the melt to the metering section, this premise is invalid. In some designs that did not have an increased barrier lead, the primary channel width is reduced to allow for the auxiliary channel. (Shippers, U.S. Pat. No. 3,701,512).
The screw lead plays an important role in the available melting area. A longer lead provides greater melting area due to fewer flights and thus contribute to melting efficiency. Screws with flights having longer leads have the ability to convey higher viscosity resins with greater efficiency than lower viscosity resins. Melting efficiency is also conveying inefficiency. Inefficiency in conveying encourages melting as the downstream force component is reduced and the rotational force component is increased, forcing resin to transfer from the primary channel into the auxiliary channel by crossing the barrier flight. In the feed and metering sections, generally square pitch is the most efficient for conveying and pressure development. In the melting or transition section, conveying is determined by the rate of melting and the pressure development capabilities of the feed section. Therefore, choice of the correct lead plays an important role in melting efficiency.
The angle of the primary channel established by the feed depth and the end depth of the primary channel also are important to the melting efficiency. In the prior art, the primary channel tapered to a minimum depth. This was intended again to emulate a conventional design, where the solids bed decreases in width and the melt pool increases in width and the melting is ideally completed at the end of the transition or melting section. While this type of melting is necessary in a conventional design, in a barrier design, separation of the melt from the solids is the object. It is not necessary or desirable to complete the melting at the end of the primary channel. It is desirable to have a controlled amount of solids at the end of the primary channel that maximizes the melting area and allows for a controlled amount of solids to enter the metering section. This increases the viscosity for greater pressure generation in the metering section.
Pressure development in the feed section is critical to melting performance because the proper compacting of the solids bed evacuates the air in the pellet or powder feed. Increasing the density of the solids bed provides the maximum heat transfer and therefore maximum rate of melting. Computer simulations provide valuable information on solids conveying and pressure development, but the accuracy is lacking.
Maximum torque in a barrier screw design normally corresponds to the maximum rate of conveying. If the solids bed is not fully compacted, maximum rate will not be achieved. Also if the screw is not capable of developing the pressure required downstream, conveying will be diminished until the viscosity is lowered to the level that the pressure development is adequate. This condition is better known as surging or unstable operation and poses a serious problem to quality control and process efficiency in extrusion operations.
A need exists, therefore, for screw designs that achieve a more optimal combination of melting, mixing, and conveying efficiency, thereby improving the overall efficiency of the extrusion process.